"We said before that if the situation remained the same in Afrin we could not attend Sochi," regional official Fawza al-Yussef said.
Rebel backer Turkey is one of the sponsors of the talks in the Black Sea resort on Monday and Tuesday, along with Damascus allies Russia and Iran.
Turkey's military offensive in Afrin "contradicts the principle of political dialogue", Yussef said.
Turkey launched operation "Olive Branch" on January 20 against the Syrian Kurdish People's Protection Units (YPG) militia in Afrin, supporting Syrian opposition fighters with ground troops and air strikes.
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The Sochi talks come after multiple failed rounds of UN- brokered talks to end Syria's seven-year war.
Yesterday, Syria's main opposition group, the Syrian Negotiation Commission, also said it would not attend the negotiations.
More than 340,000 people have been killed and millions displaced since Syria's war began in 2011.
The conflict began with the brutal repression of anti- government protests, but has since evolved into a complex war including jihadists and foreign powers.